Reduced-size cable designs are requiring ever smaller buffer tubes. As buffer tubes become increasingly small, however, excess fiber length (EFL) becomes a significant problem. As will be known by those having ordinary skill in the art, EFL can occur as a result of buffer-tube shrinkage during processing and thereafter as post-extrusion shrinkage (PES).
Various buffering processes have been disclosed with respect to loose buffer tubes to control the relative lengths of optical fibers and their surrounding tubes (i.e., fiber-to-sheath length ratio). For example, reducing buffer-tube shrinkage can be achieved by slowing processing speeds, thereby providing a larger cooling window, or by increasing buffer-tube cooling via improved cooling systems.
Exemplary patents with respect to controlling fiber-to-sheath length ratios include: U.S. Pat. No. 4,893,998 (Schlaeppi et al.), which is assigned to Maillefer S.A.; U.S. Pat. No. 5,372,757 (Schneider), which is assigned to Tensor Machinery, Ltd.; U.S. Pat. No. 6,634,075 (Lento), which is assigned to Nextrom Holding S.A; and U.S. Pat. No. 7,373,055 (Strong), which is commonly assigned to Draka Comteq BV. Each of these U.S. patents is hereby incorporated by reference in its entirety.
Demand for smaller optical-fiber cables (and smaller buffer tubes) intensifies the need to increase manufacturing production rates of loose buffer tubes. That said, merely increasing production rates leads to unsatisfactory results. For example, at higher line speeds, the time for cooling the buffer tube before the optical fibers couple to the buffer tube is reduced. In other words, at higher line speeds coupling of the optical fibers tends to occur too quickly. Consequently, more buffer-tube shrinkage is likely both during buffer-tube processing and after buffer-tube processing (e.g., as the buffer tube slowly cools over time on a take-up reel or is subjected to elevated temperatures during end use).
After the optical fibers are coupled to the buffer tube, any buffer-tube shrinkage increases EFL. Stated otherwise, less cooling time before coupling leads to more buffer-tube shrinkage after coupling, and thus undesirable increases in EFL.
Reducing, if not eliminating, buffer-tube shrinkage after optical-fiber coupling is desirable, because it will result in not only lower EFL but also better EFL control, especially in deployments in which buffer tubes are subjected to extreme temperature variations (e.g., during mid-span access). Therefore, there is a need to provide a method for making loose buffer tubes in a way that controls excess fiber length (EFL) and post-extrusion shrinkage (PES) yet provides high production efficiency, even during the startup or shutdown of buffering operations.